Microcavities based on multimodal interference

Bjørn Maes,John D Joannopoulos,Peter Bienstman,Roel Baets,Mihai Ibanescu

doi:10.1364/oe.15.006268

Abstract

We describe intricate cavity mode structures, that are possible in waveguide devices with two or more guided modes. The main element is interference between the scattered fields of two modes at the facets, resulting in multipole or mode cancelations. Therefore, strong coupling between the modes, such as around zero group velocity points, is advantageous to obtain high quality factors. We discuss the mechanism in three different settings: a cylindrical structure with and without negative group velocity mode, and a surface plasmon device. A general semi-analytical expression for the cavity parameters describes the phenomenon, and it is validated with extensive numerical calculations.

Highlights

Optical microcavities with high quality factors and small modal volumes are used in a wide range of applications such as low-threshold lasers, optical filters, nonlinear optics and cavity quantum electrodynamics [1]
The existence of high-quality cavity modes in waveguides with a zero group velocity point was shown in [2]
We show that the mechanism is very similar to the phenomenon in [3, 4], and we point out the connection with the multipole cancelation mechanism [5]

Summary

Introduction

Optical microcavities with high quality factors and small modal volumes are used in a wide range of applications such as low-threshold lasers, optical filters, nonlinear optics and cavity quantum electrodynamics [1]. The existence of high-quality cavity modes in waveguides with a zero group velocity point was shown in [2]. A zero group velocity point in the dispersion relation of a waveguide creates two modes that are intimately related. These modes are good candidates to exploit the cavity mechanism. The variation of length and frequency generates a rich variety of cavity modes, more than initially expected in [2] The parameters of these modes are precisely described by the model, which gives a straightforward expression for the eigenvalues of the (half) round-trip matrix. We examine a surface plasmon cavity device, based on a recently proposed waveguide with a zero group velocity point [8].

Semi-analytic and numerical modeling

Cylindrical cavity: negative group velocity mode

Cylindrical cavity: positive group velocity modes

Plasmonic cavity

Conclusion